When moulding articles from parisons of plastic material, certain applications may present problems in achieving the desired re-forming of the parison where the extent of the re-forming is small in comparison with the wall thickness of the parison. It may be necessary to apply very great differences in pressure to the wall of the parison, particularly in the case of parisons made from plastic material which has already been oriented. Without in any way restricting the framing of the problem, an example may be given of the requirement to produce annular protruberances in a tube-shaped parison where after the parison has finally been re-formed into the desired article the annular protruberances constitute a "neck support" ring or a "pilfer proof" ring in a bottle.
In other applications where the extent of the re-forming is great by comparison with the wall thickness and particularly in the case of plastic material which has not been oriented, existing methods enable re-forming to take place with the use of relatively small differences in pressure. When the re-forming involves extensive stretching of the material, then the wall thickness may occasionally be reduced to such an extent that the strength or the dimensional stability of the moulded article is put at risk. The only solution to this problem with existing methods has been by the use of base material of adequate thickness.
When moulding a bottle, the requirement exists to produce a neck incorporating a so-called "pilferproof" ring and/or a so-called "neck support" ring and where in certain applications the "neck support" ring is so shaped that it forms a so-called gripping ring at the same time.
One of the purposes of the gripping ring is to act as a support for the hand so that it is easier to hold the bottle firmly, for instance when it is being opened. In order to meet this requirement, the gripping ring has a diameter which is considerably greater than the diameter of the neck at the point of the neck at which the gripping ring is located. Additional cost is associated with the dimensions of the gripping ring because it increases the total amount of material required to mould the neck when existing methods are used. This is due to the over-dimensioning of the neck which is necessary in order to provide the gripping ring with adequate strength, for example when pressurized liquids such as carbonated drinks are being stored. As a rule, the gripping ring also has an undesirably large axial dimension due to the moulding problems indicated above.
One additional requirement for the rings described above is that they should exhibit dimensional stability even when heated to relatively high temperatures, for example so that filled containers may be stored at high temperatures and so that containers may be cleaned before re-use, etc. It is difficult to achieve this by existing methods without treating the material in such a way that its visual appearance changes in an undesirable manner, for example by becoming opaque, whilst the material in the remainder of the bottle remains clear and transparent.
One other disadvantage of existing methods is that in the case of coloured material those parts of the bottle where there is variation in the thickness of the material are either darker or lighter than the remainder of the bottle depending on whether there is accumulation or attenuation of the material.